1. Field of the Invention
This invention relates to an exhaust gas-purifying catalyst and a process for purifying exhaust gases. More particularly it relates to an exhaust gas-purifying catalyst using a catalyst active to the reduction reaction of nitrogen oxides with ammonia and the oxidation reaction of carbon monoxide, and a process for purifying exhaust gases using the catalyst.
2. Description of the Related Art
In recent years, in various countries including the U.S.A., a composite electricity-generating system and an electric heat-feeding system having various engines such as gas turbine, diesel engine, etc. combined with a heat recovery boiler have been installed. Since these systems have very often been installed in populated districts, nitrogen oxides (NO.sub.x) and carbon monoxide (CO) contained in exhaust gases from such systems have been a cause of environmental pollutions. Thus, exhaust gases-purifying systems as illustrated in FIG. 6 and FIG. 7 of the accompanying drawings have been employed to remove NO.sub.x and CO. In FIG. 6, an exhaust gas from gas turbine 1 is first contacted with CO-oxidizing catalyst 5a of a noble metal which oxidizes CO contained in the exhaust gas, followed by passing through super heater 2 and vaporizer 3 each connected to steam turbine 7, contacting with denitration catalyst 5b together with NH.sub.3 sprayed from ammonia (NH.sub.3)-spraying means 6 to remove NO.sub.x by reduction and being discharged via evaporator 4 from chimney 8 to the atmosphere. As CO-oxidizing catalyst 5a, catalysts of platinum (Pt), palladium (Pd) or the like supported on alumina have been used and as denitration catalyst 5b, denitration catalysts of titanium oxide containing vanadium (V), molybdenum (Mo), tungsten (W) or the like as an active component, similar to exhaust gas-denitration catalysts used for treating exhaust gases from boilers have been used.
According to the exhaust gas-purifying system of FIG. 6, since two reactors are required, there are drawbacks that the installation cost is high and the installation space is restricted. Further, since the spraying of NH.sub.3 cannot be carried out in front of the CO-oxidizing catalyst of noble metals, there are problems that a sufficient space for mixing NH.sub.3 with the exhaust gas cannot be provided between the NH.sub.3 -spraying means 6 and the denitration catalyst 5b and a high percentage denitration cannot be obtained; and an amount of unreacted NH.sub.3 is discharged. Further, there is a drawback that catalysts of expensive noble metals are used in a large quantity.
FIG. 7 is directed to an exhaust gas-purifying system of prior art having the CO-oxidizing catalyst 5a and the denitration catalyst 5b provided in the same reactor. According to such a system, the noble metal catalyst 5a usually used as CO-oxidizing catalyst is also active to NH.sub.3 -oxidizing reaction, and oxidizes NH.sub.3 in advance of the NO.sub.x reduction at the denitration catalyst 5b to generate NO.sub.x ; hence it is impossible to obtain a high percentage denitration. Further, since noble metal catalysts are used within a low temperature region, it is necessary to increase the quantity of the catalysts used; hence there is a drawback of high cost.